Clashes between anti-US fighters and interim government forces erupted in the centre of Mosul, with the two sides exchanging automatic gunfire and rockets. They were particularly heavy close to the police headquarters in the Zanjali area.
As the clashes intensified, local mosques were heard broadcasting a call to "resistance against occupation forces".
The national guard had deployed its personnel in several districts in Mosul mid-morning, as well as alongside the river Tigris and in the west of the city, the reporter said.
Two police stations were retaken in the centre and the north, and members of the national guard were seen patrolling several districts armed with anti-tank missiles and assault rifles.
The national guard also set up roadblocks in key areas of the city, which came under attack from armed groups. At 1:30pm (1030 GMT), a roadblock in the Al-Qadisiya section of the city came under heavy gunfire and witnesses reported at least one national guard member killed.
Meanwhile, a convoy of 12 armoured US vehicles started to patrol in the centre and the north for the first time since the recent bout of fighting broke out.
The US military has admitted it is
battling fighters on Mosul streets
Anti-US fighters, who had deployed in force on Thursday, taking up positions in front of public buildings, had disappeared from the streets. However, they were still seen moving around in cars at break-neck speed.
The US military acknowledged it had been involved in intense fighting with the fighters, who said they were trying to draw resources from the assault on Falluja. But the military said in a statement that US troops had not withdrawn.
"Three days ago, a number of groups ranging in size from 15-50 AIF (anti-Iraqi force) insurgents moved through the city conducting attacks on police stations, Iraqi government facilities and other fixed sites," it said.
"During this time the fighting was intense, but ... many of the insurgents were defeated. Sporadic fighting followed on Friday and Saturday," it added.
"Only a small number of terrorists are operating in the city ... and resistance is sporadic."
An AFP reporter said he saw the US military take up a position on one of five city bridges crossing the Tigris.
The military said it had killed three anti-US fighters as it took a bridge in the south after coming under small arms and rocket-propelled grenade fire.
Iraq police were nowhere to be
found during Thursday's fighting
The Iraqi-led operation in this city of more than one million people came a day after interim Prime Minister Iyad Allawi said that Iraqi special forces had arrived in Mosul and would soon move to re-impose order.
"We will be moving in the next day or so in Mosul to restore the rule of law," he had said, without giving details.
Meanwhile, Iraqi television in Mosul was back on the air after two days of interruption. The local television station - Al-Iraqiya-Ninevah - came under intense RPG fire on Thursday.
On Sunday afternoon, the station was appealing for assistance from the interim Iraqi government.
Mosul journalist Abd Allah Ghafar contributed to this report